The present invention relates to generally a self-scanning light-emitting device, particularly to a method of designing a mask pattern used in forming metal lines for a self-scanning light-emitting device.
A light-emitting device in which a plurality of light-emitting elements are arrayed on the same substrate is utilized as a light source of a printer, in combination with a driver circuit. The inventors of the present invention have interested in a three-terminal light-emitting thyristor having a pnpn-structure as an element of the light-emitting device, and have already filed several patent applications (see Japanese Patent Publication Nos. 1-238962, 2-14584, 2-92650, and 2-92651.) These publications have disclosed that a self-scanning function for light-emitting elements may be implemented, and further have disclosed that such self-scanning light-emitting device has a simple and compact structure for a light source of a printer, and has smaller arranging pitch of thyristors.
The inventors have further provided a self-scanning light-emitting device having such structure that an array of light-emitting thyristors having transfer function is separated from an array of light-emitting thyristors having writable function (see Japanese Patent Publication No. 2-263668.)
Referring to FIG. 1, there is shown an equivalent circuit diagram of a fundamental structure of this self-scanning light-emitting device. According to this structure, the device comprises transfer elements T1, T2, T3, . . . and writable light-emitting elements L1, L2, L3, . . . , these elements consisting of three-terminal light-emitting thyristors. The structure of the portion of an array of transfer elements includes diode D1, D2, D3, . . . as means for electrically connecting the gate electrodes of the neighboring transfer elements to each other. VGK is a power supply (normally 5 volts), and is connected to all of the gate electrodes G1, G2, G3, . . . of the transfer elements via a load resistor RL, respectively. Respective gate electrodes G1, G2, G3, . . . are correspondingly connected to the gate electrodes of the writable light-emitting elements L1, L2, L3, . . . A start pulse xcfx86s is applied to the gate electrode of the transfer element T1, transfer clock pulses xcfx861 and xcfx862 are alternately applied to all of the anode electrodes of the transfer elements, and a write signal xcfx86I is applied to all of the anode electrodes of the light-emitting elements.
The operation of this self-scanning light-emitting device will now be described briefly. Assume that as the transfer clock xcfx861 is driven to H (high) level, the transfer element T2 is turned on. At this time, the voltage of the gate electrode G2 is dropped to a level near zero volts from 5 volts. The effect of this voltage drop is transferred to the gate electrode G3 via the diode D2 to cause the voltage of the gate electrode G3 to set about 1 volt which is a forward rise voltage (equal to the diffusion potential) of the diode D2. On the other hand, the diode D1 is reverse-biased so that the potential is not conducted to the gate G1, then the potential of the gate electrode G1 remains at 5 volts. The turn on voltage of the light-emitting thyristor is approximated to a gate electrode potential+a diffusion potential of PN junction (about 1 volt.) Therefore, if a high level of a next transfer clock pulse xcfx862 is set to the voltage larger than about 2 volts (which is required to turn-on the transfer element T3) and smaller than about 4 volts (which is required to turn on the transfer element T5), then only the transfer element T3 is turned on and other transfer elements remain off-state, respectively. As a result of which, on-state is transferred from T2 to T3. In this manner, on-state of transfer elements are sequentially transferred by means of two-phase clock pulses.
The start pulse xcfx86s works for starting the transfer operation described above. When the start pulse xcfx86s is driven to a low level (about 0 volt) and the transfer clock pulse 02 is driven to a high level (about 2-4 volts) at the same time, the transfer element T1 is turned on. Just after that, the start pulse xcfx86s is returned to a high level. Assuming that the transfer element T2 is in the on-state, the voltage of the gate electrode G2 is lowered to almost zero volt. Consequently, if the voltage of the write signal xcfx86I is higher than the diffusion potential (about 1 volt) of the PN junction, the light-emitting element L2 may be turned into an on-state (a light-emitting state).
On the other hand, the voltage of the gate electrode G1 is about 5 volts, and the voltage of the gate electrode G3 is about 1 volt. Consequently, the write voltage of the light-emitting element L1 is about 6 volts, and the write voltage of the light-emitting element L3 is about 2 volts. It follows from this that the voltage of the write signal xcfx86I which can write into only the light-emitting element L2 is in a range of about 1-2 volts. When the light-emitting element L2 is turned on, that is, in the light-emitting state, the amount of light thereof is determined by the amount of current of the write signal xcfx86I. Accordingly, the light-emitting elements may emit light at any desired amount of light. In order to transfer on-state to the next element, it is necessary to first turn off the element in on-state by temporarily dropping the voltage of the write signal xcfx86I down to zero volts.
According to the circuit shown in FIG. 1, the cathodes of the light-emitting thyristors are connected to the ground, but it is apparent for those who skilled in the art that the anodes may be connected to the ground by opposing the polarity of the circuit.
In the self-scanning light-emitting device in which a transfer part and a light-emitting part are separated, the structure of the thyristor of the transfer part are substantially the same as that of the thyristor of the light-emitting part. Therefore, the thyristor used as the transfer element in the transfer part emits the light in its on-state. At this time, a large current as in the thyristor of the light-emitting part is unnecessary to be applied to the thyristor of the transfer part to drive it, i.e. a small fraction of current to be applied to the thyristor of the light-emitting part is applied. Therefore, the light output of the thyristor of the transfer part is smaller than that of the thyristor of the light-emitting part. As the timing of light emission in the thyristors of the transfer part is different from that in the thyristors of the light-emitting part, the light emission of the thyristors of the transfer part becomes a noise for the light emission of the light-emitting part when the self-scanning light-emitting device is used as a light print head. In order to suppress the noise, a light-shielding is required for the transfer part. The thyristors of the transfer part essentially have the structure for preventing the light from emitting outward. That is, the light is shielded by a metal (Al) line for applying a clock pulse. In this case, in order to make light shielding effective, it is required that the Al line is formed so as to be overlapped with the gate electrodes. The Al line is formed by depositing Al film and patterning it using a mask pattern in an etching process.
The thickness of the Al film is selected so thick as about 1 xcexcm in order that the Al line is not discontinued at the steps on the surface of the elements. When such thicker Al line is patterned, the resulting size of patterned Al line varies widely, because the side etching to the Al line proceeds. Taking account of this variation of the size of resulting Al line and a mask pattern misalignment, it is required that a mask pattern size is designed in such a manner that the gap is not caused between the Al line and the gate electrode.
On the other hand, in the thyristors of the light-emitting part, the electrode for applying a write signal covers a part of light-emitting surface. Therefore, the structure is required such that the decrease of luminous efficiency is small and the light-emitting surface has a uniform light emission. For this purpose, an Au electrode is made ohmic contact with the center of the light-emitting surface of the thyristor, all of the light-emitting surface is covered by a transparent insulating film, a contact hole is formed in the insulating film, and the Al line is provided on the insulating film including the contact hole.
According to this structure, as the Au electrode is provided at the center of the light-emitting surface, the light emission at the light-emitting surface is uniform. Therefore, the decrease of the luminous efficiency may be prevented by decreasing an area of the Au electrode, for example 5 xcexcmxc3x975 xcexcm square with respect to 20 xcexcmxc3x9720 xcexcm square of the light-emitting surface.
As the thickness of the Au electrode may have about 0.1 xcexcm thickness , the electrode can be patterned with good accuracy (for example, less than xc2x10.5 xcexcm). However, the thickness of the Al film is selected to about 1 xcexcm so that the patterned Al line is not discontinued at the steps on the surface of the elements as described above. The patterning accuracy for such thicker Al film is not good (for example, about xc2x11 xcexcm). This is because of side etching to the Al line during an etching process. Due to the variation of the width of resulting Al line, the width of the Al line may be larger than that of the Au electrode. In this case, the light output is decreased because of light shading by the Al line. In particular, when the self-scanning light-emitting device is used for a optical printer head, such decrease of the light output is a principal factor for the dispersion of light output among the light-emitting elements.
An object of the present invention is to provide a method of designing an optimum mask pattern for forming a metal line by an etching process, the metal line also effectively serving as a light shielding layer.
Another object of the present invention is to provide a method of designing a mask pattern for forming a metal line by an etching process so that the metal line has a width smaller than that of an ohmic electrode and larger than that of a contact hole.
According to the first aspect of the present invention, a method of designing a mask pattern for a self-scanning light-emitting device is provided. The self-scanning light-emitting device comprises a self-scanning transfer element array having such a structure that a plurality of three-terminal transfer elements each having a first control electrode for controlling threshold voltage or current are arranged, the first control electrodes of the transfer elements neighbored to each other are connected via first electrical means, a power supply line is connected to the first control electrodes via second electrical means, and a first metal line is connected to a first electrode which is one of two electrodes except the first control electrode of each of the transfer elements; a light-emitting element array having such a structure that a plurality of three-terminal light-emitting elements each having a second control electrode for controlling threshold voltage or current are arranged, the first control electrodes of the light-emitting elements are connected to the second control electrodes of the transfer elements by a second metal line, respectively, and a write signal metal line for applying a write current to a second electrode which is one of two electrodes except the second control of each of the light-emitting elements is provided; the first and second metal lines and the write signal metal line being formed by an etching process.
In this method, assuming that a mask pattern for forming the first metal line on a transparent insulating film has a width of xe2x80x9cL1xe2x80x9d overlapped with the first control electrode in a direction perpendicular to an array direction of the transfer elements, xe2x80x9cL1xe2x80x9d is selected so as to satisfy the following relation,
L1 greater than (S+dS)+a
wherein xe2x80x9cSxe2x80x9d is the distance of side etching of the first metal line, xe2x80x9cdSxe2x80x9d is the dispersion of the distance of the side etching, and xe2x80x9caxe2x80x9d is the misalignment of the mask pattern.
According to the second aspect of the present invention, a method of designing a mask pattern for a self-scanning light-emitting device is provided. The self-scanning light-emitting device comprises a self-scanning transfer element array having such a structure that a plurality of three-terminal transfer elements each having a first control electrode for controlling threshold voltage or current are arranged, the first control electrodes of the transfer elements neighbored to each other are connected via first electrical means, a power supply line is connected to the first control electrodes via second electrical means, and a first metal line is connected to a first electrode which is one of two electrodes except the first control electrode of each of the transfer elements; a light-emitting element array having such a structure that a plurality of three-terminal light-emitting elements each having a second control electrode for controlling threshold voltage or current are arranged, the first control electrodes of the light-emitting elements are connected to the second control electrodes of the transfer elements by a second metal line, respectively, and a write signal metal line for applying a write current to a second electrode which is one of two electrodes except the second control of each of the light-emitting elements is provided; the first and second metal lines and the write signal metal line being formed by an etching process.
In this method, assuming that a mask pattern for forming the write signal metal line connected to the second electrode via a contact hole opened in a transparent insulating film is assumed to have a width of xe2x80x9cW1xe2x80x9d in an array direction of the light-emitting elements, xe2x80x9cW1xe2x80x9d is selected so as to satisfy the following relation,
W+2(S+dS)+a greater than W1 greater than C+2(S+dS)+2a
wherein xe2x80x9cWxe2x80x9d is a width of the second electrode in an array direction of light-emitting elements, xe2x80x9cCxe2x80x9d is the width of the contact hole in an array direction of the light-emitting elements, xe2x80x9cSxe2x80x9d is the distance of side etching of the write signal metal line, xe2x80x9cdSxe2x80x9d is the dispersion of the distance of the side etching, and xe2x80x9caxe2x80x9d is the misalignment of the mask pattern.